Naloxone-sensitive inhibition of nicotinic transmission in the superior cervical ganglion of the cat

Long-term depression of a sympathetic ganglionic response to opioids by prolonged synaptic activity and by phorbol esters

We studied the effect of the adenylate cyclase activator forskolin, of protein kinase C-activating phorbol esters and of prolonged preganglionic input activation on the inhibitory response of the perfused superior cervical ganglion of the cat to exogenous met-enkephalin (Met-ENK). Met-ENK inhibited, in a concentration-dependent manner, the postganglionic compound action potential evoked by cervical sympathetic trunk stimulation. The inhibition was reversible, was blocked by naloxone as well as by pertussis toxin and showed no homologous desensitization in the concentration range 0.01-10 microM. Pretreatment of the ganglion with 4 beta-phorbol 12,13-dibutyrate or 4 beta-phorbol 12,13-diacetate depressed the Met-ENK response for several hours, while pretreatment with forskolin had no effect. This action of phorbol esters was prevented by the protein kinase inhibitor H-7 but not by the calmodulin antagonist W-7 or the protein kinase A inhibitor HA 1004 and was calcium-dependent. Recovery of the response from the depression produced by phorbol esters was not affected by a protein synthesis inhibitor. A 40 Hz 20 min stimulus train to the cervical sympathetic trunk mimicked the effect of phorbol esters, depressing for several hours the inhibition produced by Met-ENK. Stimulus trains of duration shorter than 5 min or frequency lower than 5 Hz were ineffective. This effect of prolonged preganglionic stimulation occurred even when the stimulus train was delivered during complete block of nicotinic and muscarinic ganglionic transmission but was lost when the stimulus train was delivered during perfusion with calcium-free Krebs. The protein kinase inhibitor H-7 prevented the depression of the Met-ENK response by the train, while W-7 and HA 1004 had no effect. These findings suggest that, in the superior cervical ganglion of the cat, a kinase, activated by phorbol esters and inhibited by H-7, exerts a long-term control of the ganglion cell responsiveness to opiate receptor activation. A similar mechanism can be synaptically activated by a non-cholinergic transmitter, released by the preganglionic axons during prolonged, high frequency, activity.

Penicillin-G induced interictal activity increases both opioid peptide tissue content and in vitro release in the rat brain

Penicillin-G has been used as a common agent to produce epileptic foci and interictal activity. The development of the interictal spikes has been associated with enhanced inhibitory effects. There is evidence that the opioid peptides play an important role in the production of some transient postictal behaviors. In order to test whether enkephalins are involved during the interictal activity, we analyzed immunoreactive met- and leu-enkephalin content and their release in vitro, after the injection of 50 IU of penicillin-G into the left amygdala. Male Wistar rats were injected once daily for 5 days, and sacrificed by decapitation (15 min after the penicillin-G infusion) on the fifth day. The rats were divided into two groups: 1. In one group we analyzed the tissue content of enkephalins in hypothalamus, hippocampus, amygdala, striatum and cerebral cortext. 2. The second group was used for the assessment of the in vitro release of enkephalins from amygdala slices. In the amygdala, the drug treatment produced an increase in the tissue content of IR-ME. No changes occurred in the other structures. The content of IR-Leu-enkephalin increased in all structures analyzed except the cerebral cortex. In vitro release of both enkephalins increased in drug treated animals. These results suggest that the enkephalins could be involved in postictal mechanisms, as a result of repetitive interictal spiking.

The site of the inhibitory action of endogenous opioids in the superior cervical ganglion of the cat

A low-frequency stimulus train to the preganglionic input inhibits synaptic transmission in the superior cervical ganglion (SCG) of the cat. The inhibition is blocked by naloxone as well as by selective antagonists at mu and delta opiate receptors, which suggests that the mediator is an endogenous opioid [27,29]. Exogenous opioid peptides, including methionine-enkephalin (Met-Enk), which is present in preganglionic axons of the SCG, inhibit ganglionic transmission by a naloxone-sensitive mechanism. In the present study we test, in the anesthetized cat, whether the naloxone-sensitive synaptic inhibition is mediated by a pre- and/or post-synaptic mechanism. As a test of presynaptic inhibition, we measured the acetylcholine (ACh) released by preganglionic stimulation into the venous effluent of the perfused SCG. As a test of post-synaptic inhibition, we measured the effect of a preganglionic conditioning train on the ganglion cell firing evoked by ganglion-stimulant drugs injected into the arterial supply of the ganglion. In presence of naloxone (3 microM), which blocked the synaptic inhibition, the amount of ACh released by stimulated preganglionic axons did not change. Thus, the endogenous opioid which mediates the naloxone-sensitive inhibition does not act by depressing ACh release. In contrast, the ganglion cell firing evoked by ganglion-stimulant drugs was markedly depressed by a conditioning train, and naloxone blocked the depression, which suggests that the endogenous mediator of the naloxone-sensitive inhibition acts postsynaptically to decrease the excitability of ganglion cells. Exogenous Met-Enk depressed both ACh release by preganglionic stimulation and the firing of ganglion cells evoked by ganglion-stimulant drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic interactions in a newly identified excitatory synapse of Helix aspersa: concurrent enkephalinergic and GABAergic modulation

Mollusc neurons, including those from Helix aspersa, have been extensely used for studies on synaptic plasticity and for pharmacological experiments. Synaptic interactions in H. aspersa are less well documented. In this work, I describe the identification and electrophysiological characteristics of a chemical synapse which is modulated by Met-Enkephalin (ME) and GABA. Low frequency repetitive presynaptic stimulation leads to a progressive amplitude reduction of the EPSPs (homosynaptic depression) of 45% after 20 stimuli and up to 80% after long-lasting repetitive stimulation (150-200 stimuli). The values of the smallest detectable EPSP after profound depression under normal neurotransmitter release levels corresponded to those obtained under high Mg(2+)-low Ca2+ conditions (0.33 +/- 0.045 mV) and to the spontaneous EPSPs (0.4 mV, sigma 2 = 0.02). Recovery after homosynaptic depression could occur by withdrawing the stimulation during 25 s after 15-20 stimuli or 2 min after 150-200 stimuli. Both, ME and GABA reduced the amplitude of the evoked EPSPs and augmented homosynaptic depression although this inhibition was independent of the homosynaptic depression trend. The amplitude of the spontaneous EPSPs was not affected. Naloxone alone did not produce any effect, making it unlikely that the enkephalinergic system could be recruited during homosynaptic depression. ME also shortened the duration of the presynaptic action potential and enhanced its afterhyperpolarization. These results suggest that ME and GABA exert a presynaptic modulation.

Lack of long-term potentiation, non-cholinergic transmission and muscarinic inhibition in cat superior cervical ganglia innervated by nodose ganglion cells

In anesthetized cats, in which a nodose-superior cervical ganglion (SCG) anastomosis had been performed 6-9 months earlier, the nictitating membrane contraction evoked by electrical stimulation of the cervical vagus nerve ipsilateral to the anastomosis was recorded. The competence of nodose neurons in regulation the multiple synaptic mechanisms of the sympathetic ganglion was tested by comparing this response with the responses to stimulation of (self-reinnervated SCG). The response of the nictitating membrane ipsilateral to the anastomosis was smaller and had a lower EC50 for hexamethonium (C6) than the responses of the nictitating membrane ipsilateral to the intact or sutured CST. A 40 Hz 10s stimulus train to the intact or sutured CST produced potentiation of ganglionic transmission lasting 1 hour or longer, while a similar stimulus train to the anastomosed cervical vagus nerve produced no potentiation. During block of ganglionic nicotinic transmission with C6, CST or vagus nerve stimulation evoked responses which increased in amplitude with increasing stimulus frequency and were blocked by the selective muscarinic receptor antagonist pirenzepine. When the anticholinesterase eserine was added, the responses evoked by preganglionic stimulation decreased in amplitude in the intact SCG, as previously shown [7], and in the self-reinnervated SCG. This effect, which is due to inhibition mediated by muscarinic receptors selectively blocked by AF-DX116, was absent in the anastomosed SCG. During block of ganglionic transmission with C6 and atropine, a 40 Hz stimulus train to the intact or to the sutured CST evoked a slow, small amplitude contraction that was enhanced by naloxone. This response, most likely mediated by peptides [6], was absent in the anastomosed SCG.(ABSTRACT TRUNCATED AT 250 WORDS)

The receptors activated by exogenous and endogenous opioids in the superior cervical ganglion of the cat

Nicotinic transmission in the superior cervical ganglion of the cat was studied in vivo and in vitro by recording the postganglionic compound action potential evoked, under partial block with hexamethonium, by supramaximal 0.2 Hz stimulation of the cervical sympathetic trunk. The compound action potential was depressed following a stimulus train (5 Hz, 40 s) applied to the same set of axons. The inhibition was antagonized by naloxone, suggesting mediation by endogenous opioids. Agonists selective for mu-, delta- and kappa-opioid receptors, injected into the arterial supply of the in situ ganglion or added to the perfusion fluid in the in vitro ganglion, also produced a naloxone-sensitive inhibition of the compound action potential. The synaptic inhibition was antagonized by the delta-selective antagonist ICI 174,864 and by the mu-selective antagonist CTAP, but not by the kappa-selective antagonist Nor-BNI. These results suggest that all three main types of opioid receptors are present in the ganglion but only mu- and delta-receptors are involved in the inhibition of ganglionic transmission mediated by the endogenous opioids.

Use-dependent fade and two-rate recovery of a naloxone-sensitive inhibition in the cat superior cervical ganglion

In anaesthetized cats, a short train to the cervical sympathetic trunk (CST), of frequency between 0.5 and 5.0 Hz, inhibited the postganglionic compound action potential (CAP) evoked by a test shock to the CST. When the CST was split into two bundles, the inhibition was obtained both homosynaptically and heterosynaptically. The inhibition was antagonized by naloxone and enhanced by peptidase inhibitors. Leu-enkephalin (ENK), injected into the arterial supply of the superior cervical ganglion (SCG) depressed the CAP. The ENK-evoked inhibition was antagonized by naloxone, enhanced by peptidase inhibitors and was occluded by the train-evoked inhibition. After prolonged stimulation of the CST (5-20 min at 5 Hz), the inhibition produced by a short train was depressed (faded). The fade was greater the longer the stimulation. After 20 min of stimulation the inhibition was absent. At this time the depressant effect of ENK was unchanged. When only one bundle of the split CST was stimulated at 5 Hz for 20 min, only the inhibition produced by that bundle faded, although both bundles converged onto a common set of ganglion cells. Thus, the fade is not due to receptor desensitization but more likely to failure to release the inhibitory mediator by the overstimulated axons. When, after 20 min of 5 Hz stimulation, no further stimuli were applied, the inhibition recovered in 60 min (fast recovery). However, recovery was slower when stimulation lasted longer than 20 min. Recovery took three days when stimulation lasted 80 min (slow recovery). The inhibition was markedly depressed after application of colchicine to the CST three days earlier.(ABSTRACT TRUNCATED AT 250 WORDS)